1. Field of the Invention
This invention relates to a plastic valve and more particularly to a butterfly-type valve having a plastic body with an actuator externally supported on the valve body so as to distribute the actuating forces uniformly over the exterior surface of the valve body and prevent the application of actuating forces at a localized point on the valve body.
2. Description of the Prior Art
It is known to fabricate a butterfly-type valve of molded polyethylene and form a unitary valve body that includes a conduit portion for rotatably supporting a valve disc or closure member and a neck portion extending upwardly from the conduit portion for rotatably supporting the valve stem and a valve actuator. The valve conduit portion is molded in surrounding relation with an annular valve seat. The valve seat is embedded in the valve body during the molding process. The valve disc is positioned in the passageway of the conduit portion for movement between an open position aligned with the direction of flow through the valve and a closed position aligned at right angles to the direction of flow. In the closed position, the peripheral edge of the disc tightly engages the valve seat to effect a seal.
The valve disc is nonrotatably mounted on a valve stem which is rotatably supported in the valve body. The stem extends upwardly from the valve disc through the conduit portion and the neck portion of the valve body. The conduit portion and the neck portion are integrally connected during the molding process and a stem opening extends through the conduit portion and the neck portion. Therefore, the neck portion is exposed to the fluid line pressure and forms, with the conduit portion, a pressure boundary within which the fluid must be contained. The neck portion must have a sufficient cross-sectional area to accommodate seals around the valve stem to contain the line pressure. Also, the body of the neck portion must have a sufficient cross-sectional area to withstand the torsional forces transmitted by the valve actuator during operation of the valve.
The integral neck portion of the valve body not only supports the stem that extends upwardly through a bore in the neck portion but also supports the valve actuator. The valve actuator is positioned in surrounding relation with the exterior surface of the neck portion. A known valve actuator includes a cap that is molded onto the upper end portion of the valve stem and extends downwardly therefrom into surrounding relation with the neck portion. The valve stem is movably rotated through a one-quarter turn to move the valve disc between the open and closed valve positions.
The actuator cap is provided with a directional arrow for indicating the direction for turning the valve to the open or closed position. For the open position the indicator is in line with the direction of flow through the conduit portion. In the closed position the indicator is normal to the direction of flow through the conduit portion. To limit the movement of the actuator cap through an arc of 90.degree., the cap is provided with stops that are movable into and out of engagement with stop surfaces provided on the periphery of the valve neck.
The torque applied to the actuator when the actuator is turned to fully open or fully close the valve is transmitted to the valve neck. Therefore, the valve neck must have a structural strength capable of withstanding the torsional loads generated during operation of the valve. In view of the fact that the polyethylene material from which the valve body is fabricated is a relatively soft and medium yield material, the valve neck must have a substantial wall thickness to withstand the torsional loads from the actuator. Consequently, the wall thickness of the neck must be substantially greater than the wall thickness of the conduit portion of the valve. Increased size of the valve neck is also necessitated by the need for the valve neck to accommodate the bore for the stem and the seals around the stem. The valve neck forming part of the pressure boundary within the valve must meet the rigid design and quality control requirements imposed on the valve body.
It is the conventional practice to fabricate the above described plastic valve by an injection molding process. In view of the fact that the neck portion of the valve has specific structural requirements as above enumerated, the valve mold must be particularly manufactured to form the desired valve neck configuration. This has the undesirable consequence of increasing the cost of fabricating the valve body mold. Because the wall thickness of the neck portion is greater than the wall thickness of the other parts of the valve, a greater volume of plastic material is required to form the valve neck. This increases the overall time for the injected valve body to cool and solidify before the mold can be opened and the formed valve extracted.
A further undesirable consequence of the particular requirements for the formation of an integral valve neck-valve body design is the high tooling cost associated with the specialized valve mold and the somewhat complex machining operations required in making the valve mold. As a consequence the above factors contribute to substantially increasing the time and cost of fabricating a plastic butterfly valve having an integral neck and valve body.
While it has been suggested to fabricate a butterfly-type valve from plastic material such as polyethylene, the known prior art device having an integral valve neck and valve body requires that the valve neck have sufficient structural strength to withstand the torsional forces imparted thereto by the actuator. The fabrication of such a valve is rather expensive and time-consuming. Therefore, there is need for a butterfly-type valve fabricated of a plastic material and having means for supporting the valve actuator in such a manner to withstand the torsional loads imparted by the actuator and permit economical and efficient fabrication and assembly of the valve.